The isolation and biochemical characterization of a major opsonic protein from rat serum have been partically achieved. The protein was identified as an alpha 2 macroglobulin in (MG). Unlike the human alpha 2-MG this protein did not bond trypsin. We plan to purify a similar opsonic from human sera and determine its properties in relation to the rat opsonic. Specificity studies suggest that the rat protein is involved in the clearance of tissue fragments which could originate in surgery of trauma. We also want to establish whether the human opsonic is related to a tumor necrotic factor recently isolated from mouse serum. We are in the process of determining the role of heparin which is required as a cofactor for opsonization. Finally we want to study the site of syntheses of the alpha 2-MG in the rat. BIBLIOGRAPHIC REFERENCES: C.R. Waltenbaugh, C. Allen, J. Molnar, T. Sabet and P. Van Alten. Impairment of clearance by the reticuloendothelial system of bursectomized chickens. J. Reticuloendothelial Soc. 19, 3-9 (1976). S. McLain, J. Siegal, J. Molnar, C. Allen and T. Sabet. A phagocytosis promoting factor of rat serum independent of the complement system. J. Reticuloendothial Soc. 19, 127-138 (1976).